The invention relates to process control, and particularly to methodology that reduces CD (Critical Dimension) non-uniformity in resist processing for semiconductor mask or wafer manufacturing, and compensates interfield signature from other processes.
FIG. 1 shows a basic lithography flow. First, a wafer is prepared, and the wafer surface is cleaned (S101). Then, a thin layer of resist (photoresist) is spin coated on the wafer surface (S102). In soft bake, resist solvent is partially removed by using a heating device, such as a hot plate (S103). Then, at least one mask is aligned to the wafer, and resist on the wafer is exposed, resulting in an acid latent image (S104). In PEB (Post-Exposure Bake), a chemical reaction is thermally induced. This, being catalyzed by the acid, may be the activation of a crosslinking agent for a negative resist or the deblocking of the polymer resin for a positive resist (S105). In development, the resist that is not crosslinked or is deblocked is removed (S106). Then, in ADI (After Development Inspection), CD of the resist pattern is inspected using a SEM (Scanning Electron Microscope) to determine whether it conforms to the specification (S207). If not, the resist is removed and the whole procedure is reworked (S108). If yes, in hard bake, more resist solvent is removed (S109). Thereafter, pattern transfer, such as etching, is performed (S110). Then, the resist layer is stripped from the wafer (S111). Finally, AEI (After Etching Inspection) is performed.
For commonly used commercialized chemically amplified resist, the PEB sensitivity is of the order of 10 nm/degree. That is, change of PEB temperature by 1 degree will result in about 10 nm change of resist CD. Thus, for application of, e.g., the 90-nm node and beyond, temperature uniformity of a hot plate is controlled to be less than 0.2°, being a big challenge for track manufacturers.
Tokyo Electronic Industry Co., Ltd (TEL) uses a RTD (Resistance Temperature Detector) wafer to calibrate their chilling precision hot plate (CPHP). Since the chamber accommodating CPHP is opened in order to place the RTD wafer, the temperature measured in such a way cannot reflect the original thermal environment that a wafer experiences during its PEB. Further, it takes a long time for the RTD wafer to complete the whole calibration procedure. Additionally, since there are 29 sensors on the RTD wafer, such characterization of a hot plate will not be detailed enough.
OnWafer Technologies proposes a wireless device that mimics a wafer with built-in thermal sensors. Thus, it can be sent into the track just like a production wafer and measures the whole thermal cycle at correct thermal environment. However, if the required accuracy is very high, matching of response between different sensors should be guaranteed. If it is uniformity of CD, rather than temperature, that is to be adjusted, then the relation between CD and the whole thermal cycle needs to be established. Also, similar to the RTD wafer, there are 42 sensors on the wireless sensor wafer, still too few to give a sufficient characterization of a hot plate. Moreover, such methodology is of high cost due to short lifetime of the wireless sensor wafer.
What is pursued here is a methodology directly utilizing resist as the thermal sensor for accurate, efficient, and economical measurement of the temperature of a hot plate, so realizing process control by adjusting the hot plate, therefore improving CD uniformity.